The invention relates to the generation of acoustic energy in a fluid environment, and, more particularly, to the use of the water hammer principle to generate the acoustic energy where most of the frequency content lies in the zero to 160 Hertz band. A characteristic of the acoustic radiator is the ability to generate a direction distinctive acoustic wave adaptable for use in both marine and land environment.
In conducting seismic exploration in both marine and land areas it has been the practice to generate seismic waves by the detonation of explosive charges or by the utilization of mechanical or electrical or thermodynamic devices to generate an impact or a vibration. Each of the various types of acoustic sources has undesirable characteristics such as the tendency to generate non-signal types of acoustic wave trains generally identified as "noise" in contrast to the desired "signal". Each of the various types of acoustic sources has its own pulse generating characteristic, generally providing frequencies of little value in seismic exploration or of a frequency rich in "noise" spectrum.
All marine acoustic pulse generators have an undesirable characteristic known as the hydraulic afterflow, which generally introduces a seismic "noise" that severly distorts the desired seismic "signal". The problem of the "afterflow" is well stated in the Seismic Energy Sources 1968 Handbook prepared by Staff Members of United Geophysical Corporation, Pasadena, Calif. on Page 9.
"All types of underwater seismic energy sources inevitably give rise to hydraulic afterflow. The phenomenon is inherent in the "point" (or linear) nature of the source, and in the associated spherical (or cylindrical) divergence. Afterflow represents temporary storage of kinetic energy not immediately radiated. This energy must ultimately be dissipated or taken care of in some fashion, and all seismic energy sources must contend with this situation."
Much effort has been expended to develop or to instigate various techniques designed to remove afterflow by a cancellation process making use of a number of individual sources in various combinations. All efforts to develop an acoustic source free of these afterflow effects have been to no avail, just as predicted in the 1968 statement in the Handbook. Some success has been achieved by using more than thirty individual sources each of a different characteristic, time wise and pulse wise, such as an array of air guns, in a semirandom configuration in an expensive and complicated array.
The Energy Handbook (1968) analyzes in detail more than fifteen types of marine acoustic sources and applies the following basic acoustic approximation equations to each of these source types in the analysis: ##EQU1## v is instantaneous particle velocity p is instantaneous (excess) pressure (P-Po)
.rho. is density of water PA1 c is velocity of sound propagation in water PA1 .rho.c is acoustic impedance of water PA1 r is radial distance to measurement point PA1 t is time PA1 g is acceleration of gravity in feet per second squared; PA1 w is weight of water in pounds per cubic foot; PA1 Q is ##EQU5## K is bulk modulus of water under compression=46,080,000 pounds per square foot; PA1 E is modulus of elasticity of conduit, glass reinforced nylon=144,000,000 pounds per square foot; PA1 D=diameter of conduit in feet; and PA1 b=thickness of conduit wall in feet.
The first term is the acoustic or radiation term showing particle velocity in the acoustic pressure wave is proportional to the excess pressure generated by the source. The second term, proportional to the time integral of pressure ##EQU2## is the afterflow.
The optimum seismic acoustic source would generate a high instantaneous pressure with no outward or tangential expansion.
A purpose of the invention is to provide a means for generating an acoustic pulse to be transmitted into the surrounding fluid medium, in a marine environment, in such a manner that the hydraulic afterflow has been reduced to the vanishing point. This is achieved by containing the pressure pulse in a highly rigid but acoustically transparent conduit so that there is essentially no tangential expansion or spherical divergance.
It is well known that when a rapidly flowing stream of water in a rigid conduit is suddenly arrested by closing a valve in the conduit, the kinetic energy therein is converted to pressure energy. The pressure pulse generated by this energy conversion travels upstream away from the valve in the rigid conduit until it is reflected in total or in part by a discontinuity in the conduit, to be reflected again at the closed valve. It is the above described phenomena which is known as "water hammer".
The water hammer phenomena is discussed in great detail in many books, papers and reports. In Hydraulic Transients, McGraw Hill 1951, the author George R. Rich, explains the phenomena, develops the equations, depicts typical water hammer transients and applies the principles to various problems related to water power plant conduits. A similar analysis is presented in Fluid Mechanics, McGraw Hill 1958, by Victor L. Streeter. In these source medias the water hammer wave is shown to be a succession of square waves of pressure with each pressure pulse having a time span equal to 2 L/a, where "L" is the conduit length and "a" is the speed of the water hammer wave, each followed by a rarefaction pulse covering a similar time span. The agreement between calculated and measured pressure pulses is excellent, confirming the theoretical soundness of the water hammer theory.
The zone or local of the conversion of kinetic energy into elastic strain or pressure energy travels upstream with an acoustic wave velocity "a" leaving the water downstream behind the advancing wave front at zero velocity, but at a high pressure in a slightly expanded conduit. The pressure developed by the water hammer is ##EQU3## where "M" is the mass of water whose velocity is reduced the amount "dv" in time interval "dt".
Analysis of the water hammer phenomena in a physically realizable system, such as in this invention, shows that the pressure developed by arresting the fluid flow is ##EQU4## where V is the velocity of water flow in the rigid conduit before valve closure in feet per second;
The velocity of travel in feet per second of the water hammer wave within the conduit "a" is given by the following equation: ##EQU6##
The water hammer pressure can be expressed by the following equation, which is a combination of the two preceeding equations: ##EQU7##
The above equation shows that the pressure in the water hammer wave is proportional to fluid flow velocity, which is readily controllable by varying the magnitude of the pressure forcing the fluid to flow through the conduit. The velocity of travel of the water hammer wave "a" is dependent upon the bulk modulus of water, which is invariable, and the modulus of elasticity of the conduit material and the ratio of conduit diameter to conduit wall thickness. Thus, in a particular system, the only variable affecting the pressure developed in the water hammer wave is the velocity of flow. To develop high water hammer pressures it is necessary to have a means for supplying a high pressure to force the fluid to flow.
The equation showing the expansion of the conduit caused by the fluid under high pressure in the section behind the water hammer wave is: ##EQU8##
The above equation shows that the pressure developed by the water hammer is directly proportional to the velocity of water flow. The present invention includes provision for the use of glass fiber reinforced nylon resulting in Q of 0.0772.times.10.sup.-6. The water flow velocity is 500 feet per second, resulting in a water hammer wave travelling in the conduit at a wave speed of 2545 feet per second and a pressure of 17,240 pounds per square inch. The conduit expansion caused by this increase in pressure is approximately seven percent, or a conduit eight inches in diameter would expand approximately one half of an inch in diameter.
It is apparent that the present invention would provide a means for generating very high pressures by the water hammer phenomena with very small mass flow associated with the seven percent increase in diameter of the conduit. It is noted that the mass flow is distributed over the time of travel of the wave front from the valve to the open end of the conduit. The present invention also teaches the use of a radiating conduit which may be on the order of forty feet in length. The water hammer wave travels at a velocity of 2545 feet per second in a glass fibre reinforced nylon conduit. The acoustic pulse radiated from the water hammer wave of compression in travelling from the valve to the open end and back to the valve, a distance of 80 feet consumes thirty one milliseconds. The wave then is reflected at the valve and travels to the inlet end of the conduit and returns in the form of a rarefaction with reduced pressure approaching the vapor pressure of water, in thirty-one milliseconds. Thus, a semi-square wave results with a period of 0.62 milliseconds equivalent to sixteen Hertz. A spectral analysis of this semi-square wave shows a primary concentration of acoustic energy in the zero to 32 Hertz band up to 80 Hertz band, with essentially all the energy within the 0-160 Hertz band. One feature of the invention is the ability to control the frequency of the acoustical pulse by selection of length of conduit. The time span of the compression and rarefaction phases of one cycle of a semi-square wave is inversely proportional to the length of the conduit. The pulse generated within a radiating conduit 40 feet long has a concentration of its energy within the zero to thirty-two Hertz frequency band, if the conduit is 20 feet long the energy is concentrated in the zero to sixty-four Hertz band.
Another feature of the present invention is thus the use of conduit material that is acoustically transparent such as glass fibre reinforced nylon or Dupont Kevlar Aramid reinforced material which have very high modulus of elasticity and acoustic impedance essentially the same as water. Thus, the acoustic pulse simply passes through the walls of the acoustically transparent material of the conduit. Since the conduit does not act as a diaphragm, which is the case when a highly compliant material such as polyurethane elastomer or similar plastics material or rubber is used, there is no mass flow. The mass flow or afterflow is a function of the degree of compliance of the conduit material. Also, the magnitude of the acoustic pulse is affected by the compliance of the material, and in some cases the compliant diaphragm acts as a terminator of the water hammer wave due to the elastic discontinuity introduced by the diaphragm. In this case only the area of the opening into the diaphragm radiates acoustic energy.
The generation of water hammer waves in a unique arrangement of rigid metal conduit and various flexible diaphragms was examined in great detail by Billy Wayne Davis and the results were published in a Dissertation from Southern Methodist University, Apr. 9, 1969, entitled "A Study of the Spectral Character of Acoustic Waves Radiated by a Waterhammer Excited Cylindrical Diaphragm". The results of this study were summarized and reported in part in a paper presented by Davis at the Offshore Technology Conference, April, 1970, entitled "The Transient Character of the Near-Field Acoustic Radiation from a Cylindrical Diaphragm Excited by Waterhammer Transients".
The device disclosed by Davis to generate the water hammer waves consisted of a section of aluminum pipe whose length is denoted by Ly, then a flexible polyurethane elastomer diaphragm length L, and then another section of aluminum pipe length Lx terminated in a valve mechanism. A portion of the upstream or uppermost aluminum pipe is immersed in a water test tank, the entire diaphragm is submerged as is the lower section of aluminum pipe. Water in a pressure tank is forced through the system and a means is provided for suddenly arresting the water flow at the end of the lower section of aluminum pipe. Means are provided for measuring and recording photographically the velocity of water flow at the instant of closure of the arresting valve. Calibrated blast transducers were inserted into the wall of the aluminum pipe. One was located near the arresting valve, one was located near the lower end of the diaphragm, one near the upper end of the diaphragm and another some distance upstream from the diaphragm. The outputs of these blast transducers were displayed on an oscilloscope and recorded photographically on Polaroid Film. The acoustic pulse radiated into the water was recorded in a similar manner using a calibrated hydrophone located three feet to one side of the midpoint of the flexible diaphragm.
Davis made amplitude frequency spectrum analysis of a large number of pulse recordings. He reported, for a flow velocity of 34.6 feet per second that 67% of the total acoustic energy was in the 0-3000 Hz and less than 3% to be in the 0-100 Hz band, as shown in FIG. 25. The conclusions then set forth in this disclosure on Pages 75 and 76 include:
That acoustic energy radiated by a water hammer excited cylindrical diaphragm is distributed rather evenly over a broad frequency band 0-4000 Hz;
That peak acoustic pressure of 88 psi at 3 feet could be achieved by a flow velocity of 170 feet per second and pipe diameter of 9.00 inches; and
That the length of diaphragm, above 9.5 inches, has no appreciable effect upon energy concentration within the spectrum.
The present invention provides for generation of acoustic pulses by a water hammer pressure wave of 17,240 pounds per square inch, which is some 10 times greater than that disclosed in the aforesaid prior art reference. The pulse width is 31 milliseconds which is some 200 times broader than that of the Davis disclosure and essentially all the available energy will be in the 0-80 Hz band up to the 0-160 Hz band, whereas less than 3% of the energy in the pulse developed in the Davis system is in the 0-100 Hz band. The radiating surface in the invention is the entire circumferential surface of a conduit some 40 feet in length whereas the radiating surface in the case of Davis is only the area of the end of a metal pipe. In the case of an 8 inch conduit 40 feet long the radiating surface is 240 times greater.
Other prior art contructions have heretofore proposed utilizing the water hammer concept by providing aquatic means for generating pulses by the water hammer phenomena and detecting same in a seismic detection system. One such prior art apparatus is set forth and disclosed in U.S. Pat. No. 3,376,949 issued to Buford M. Baker and James H. Waugh, Jr. on Apr. 9, 1968 and entitled "Water Hammer Marine Seismic Source". In the embodiment of the water hammer set forth therein, the apparatus effecting the water hammer includes a tubular member having a perforated zone formed upon one end thereof for permitting escape of fluid flow energy to produce the seismic wave in the particular marine environment for which the apparatus is constructed. The tubular member itself is not constructed for pressure wave propagation. As described therein a heavy rubber tube, or the like, is clamped around the perforated region of the tubular member to encompass the perforated zone. The pressure wave energy of the flowing stream is then vented by being transmitted through the perforations in the zone. Means are therein provided for abruptly terminating the flow to permit the select pressure fluid flow through the aforesaid perforations. Baker proposes the use of a rubber tube surrounding a perforated zone in the pipe containing the fluid flow. There is no essential difference between the device disclosed by Baker and the device investigated by Davis. Both provide for a highly compliant diaphragm through which the pressure in the water hammer wave appearing at the end of the solid pipe is radiated into the surrounding fluid.
Acoustic sources used on land utilize dynamite, either placed on the surface, or to various depths below the surface. Other land acoustic sources consist of impulse generating mechanical or thermo-mechanical devices such as a dropping weight, an impact generated by air gun driven impactor, a vibrator, or by burning of combustible gases in an impactor chamber. Everyone of these various sources generates an acoustic pulse, in the case of the vibrator time varying, that is designated as the signal or desired pulse, plus a rather complete spectrum of "noise" waves such as Raleigh waves, commonly called "ground roll", and shear waves all of which are multiply reflected and scattered from the physical discontinuities in the earth's surface. These noise waves are generated primarily as a result of surface or near surface deformation of a "mass flow" or transient or permanent type.
Other references which may relate generally to this subject matter include:
______________________________________ U.S. Patent No. Date Name ______________________________________ 3,536,157 10/27/70 Anstry 2,281,751 5/05/42 Cloud 3,690,403 9/12/72 Davis 3,721,311 3/20/73 Mott-Smith 2,424,108 7/15/47 Merten 3,863,203 1/28/75 Patton et al 2,759,143 8/14/56 Arps 3,376,949 4/09/68 Baker et al ______________________________________
A purpose of this invention is to generate an acoustic impulse that is coupled to and transmitted into the earth's surface with a minimum of "mass flow" or surface deformation of a transient or permanent type and yet within a predefined frequency band. A high pressure acoustic pulse is generated in a rigid, acoustically transparent conduit which pulse of predetermined frequency is coupled to the earth's surface through a surrounding water medium in close contact with the earth's surface.
There is an almost complete absence of any "mass flow" associated with the water hammer generated acoustic pulse, thus there is essentially no "mass flow" or deformation of the earth's surface, either transient or permanent resulting from the coupling of the pulse from the generator by fluid means contained within the flexible chamber surrounding the acoustic radiator. The acoustic pulse is distributed over a considerable lateral extent due to the elongate acoustical transparent tube within which the pulse is generated. The pressure pulse in the water hammer travels horizontally in the elongate tube, thus by choice of orientation of the tube, the direction and the effective shape of the pulse can be controlled.